I would add in that you have consulted a number of professional arborists that know NIP. VERY few blow overs have occurred to their knowledge, including ones that have withstood hurricanes
Offer him the website
www.treesaregood.org.
Explain to him that "topping" has been perpetuated through disinformation, that it goes against all accepted tree care practices, and is akin to amputation with a dirty knife. The tree will never, ever "heal". It is not in the biology of the tree.
I suspect if you use Google Images for "topping wounds" (the part that people never see from the ground, until the tree fails/ is felled), you will get some good pics.
I would print some of these, along with cut and pasting some of the responses from this thread.
You may ask him how much he is prepared to budget on the management of the topping damage.
Search for TJinSC 's thread about grass under trees.
If the roots are interfering with the lawn, then he should consider dressing up the landscape with mulch and some ground covering plants. Proper mulching instead of grass is both beneficial to the plant for having the mulch, and getting rid of the grass. See the above thread.
You may wish to indicate that as such an unprofessional practice, you would not want to put your name on it, any more than a mechanic would put his name on putting sand in a gas tank. Maybe you know his profession and could make a better, close to home analogy. A barber with a bad, bad hairpiece. Susan Boyle trying to be Milli Vanilli (that may be before your time, youtube it--they were busted for lip syncing). A roofer getting paid to rip shingles off the roof here and there, and cut a few holes.
More important than the maintenance, the various damages and introduced decay will put him, his property, etc at elevated risk over time.
